1. Field
This disclosure pertains to a method of fabricating a germanium-on-insulator (GOI) substrate and a method of bonding an epitaxial germanium layer to a semiconductor substrate to form a GOI substrate.
2. Discussion of Related Art
There is an increasing interest in using silicon-germanium (Si—Ge) alloy as a material for microelectronic and optoelectronic device applications. Germanium (Ge) is known to have high carrier mobility (e.g., high hole and electron mobility) and optical absorption as compared to silicon (Si). This is one reason why Ge is useful for devices that require enhanced performance and/or high quantum efficiency. Embodiments of devices that would benefit from the use of a Ge film include metal-oxide-semiconductor (MOS) transistors, optical detectors, and other optoelectronic devices, to name a few. There is also an increasing interest in bonding a germanium layer onto an insulator that is formed on a semiconductor substrate to form the GOI because the insulator layer in the semiconductor substrate helps reducing current leakage in the semiconductor device that is formed in the germanium layer.
There are generally two methods for forming a GOI substrate. In the first method, a crystalline Ge donor wafer is transferred (or bonded) onto a semiconductor wafer handle having an insulation layer. Ion exfoliation is then used to remove portions of the Ge donor wafer to leave a Ge layer bonded to the semiconductor wafer. Using a crystalline Ge donor wafer to transfer Ge onto a wafer handle is costly. For instance, the crystalline Ge donor must be polished to the desired thickness which is time consuming and expensive. It is thus difficult and costly to remove surface non-uniformity after the ion exfoliation.
In the second method, an epitaxial Ge layer is formed on top of a semiconductor wafer handle having an insulation layer. Currently, an epitaxial Ge layer cannot be directly bonded to a wafer handle due to the high roughness inherent in the Ge layer. Direct wafer bonding requires smooth surfaces of about <0.5 nm to <1.5 nm RMS roughness. An epitaxial germanium (Ge) layer usually has a roughness of about >2 nm RM to >4 nm RMS. Such roughness in an epitaxial Ge layer makes direct bonding of the Ge layer to an insulator on a semiconductor substrate (or wafer handle) difficult. In addition, to treat the surface of the epitaxial Ge so as to provide it with a smooth surface is difficult and expensive. For example, chemical mechanical polishing of an epitaxial Ge layer surface is time consuming.
There is thus a need for a new method of forming a GOI substrate.